This invention relates to fluxes of the kind used for refinement of primary or pro-eutectic Silicon crystal grains in high-silicon aluminum alloys.
In recent years, hyper-eutectic Al-Si alloys have come to be used extensively not only as materials for engine pistons but also as those for other automotive parts including crankcases, cylinder liners and brake drums because of their excellent characteristics. Lo-called hyper-eutectic Al-Si alloys contain about 12% or over of silicon and, among others, aluminum alloy materials for casting or forging use containing from about 17% to about 25% of silicon are called high-silicon aluminum alloys and are known to exhibit highly excellent properties including: (1) a coefficient of thermal expansion lower than that of any other aluminum alloy material; (2) a Satisfactory wear resistance; and (3) a considerable resistance to heat. The practical range of use of high-silicon aluminum alloys, however, has been rather limited as, upon solidification, primary crystals of silicon grow into coarse square-shaped grains, impairing the mechanical properties, including machinability, of the resulting alloy product.
The relationships of the silicon content of Al-Si bivary alloys to their coefficient of thermal expansion and to some of their mechanical properties are graphically illustrated in FIG. 1. As observed, the thermal coefficient of Al-Si alloys linearly decreases with increase in silicon content. Obviously, the lower the thermal coefficient of the engine material is, the smaller is the amount of thermal expension of engine parts made of such material as occuring with rise in temperature of the engine in operation and the more the clearence between the piston and associated cylinder wall can be reduced to improve the engine performance. This is the reason why hyper-eutectic Al-Si alloys have come to be widely employed as materials for pistons and other engine parts. However, parts made of hypereutectic Al-Si alloys and, among others, of so-called high-silicon aluminum alloys have previously been limited in finishing accuracy because of the formation therein of coarse primary crystal grains of silicon and, under this situation, the technique for refinement of primary crystal grains in high-silicon aluminum alloys is growing particularly important.
Conventional methods of refining primary crystal grains of silicon, which principally include an addition of phosphorus, are listed below:
1. Addition of phosphorus penta-chloride (PCl.sub.5);
2. Addition of Cu-P alloys or Fe-P or Ni-P alloys;
3. Addition of elementary phosphorus or its mixture with KCl or K.sub.2 T.sub.i F.sub.6 ;
4. Addition of an element or elements other than phosphorus (for example, Z.sub.n S, F.sub.e S.sub.2, T.sub.i + Z.sub.r, V, G.sub.a + S.sub.e); and
5. Solidification by quenching or ultrasonic vibration.
The first method, (1), is rather limited in industrial use for a number of reasons including: unpleasing smell rising during the addition of PCl.sub.5 to the molten mass, remarkable hygroscopicity of PCl.sub.5 and hence handling inconvenience in its use. The method (2) is disadvantageous in that elementary metals are admixed in substantial amounts together with P. Among others, inclusion of Fe in any amount should be avoided as it impairs the quality of resulting alloy products. The method (3) is dangerous as phosphorus, when added to the molten mass, burns violently on the top thereof and disadvantageous in that any stable application of the additive is hardly feasible. As regards the method (4), any of the additive elements used therein seems to be less effective than phosphorus in view of the results of many investigations previously made and reported. Finally, the method (5) is considerably effective but lacks practicability with its range of application limited because of the volume of meet and the equipment required.
Under these circumstances, use of fluxes is considered desirable in order to attain the purpose of refining primary silicon crystal grains at all times in a stable condition. Fluxes previously employed, however, have contained as a main ingredinet a phosphorus compound with chlorine or fluorine and, when applied, unavoidably produced gases harmful to the operator and other neighboring persons. The only measure previously resorted to meet this situation has been just to manage to reduce the amount of evolving gases as far as possible.